I understand your frustration.
Pathogens tend to become resistant to continued use of antibiotics. My question is how much colloidal silver are you giving these two cats and what brand are you using. Right now, I would give them at least two ounces a day. And, of course, continue administering the probotics.
Now, I'm a bit confused with regard to the raw diet. Did you say it is canned? I would switch them to a completely raw diet. Many pets heal by just being switched to raw. I may be misunderstanding the "canned" part. The raw food should be fresh or frozen.
The only other advice I can give you is to find a holistic vet who can address this problem naturally.
My heart goes out to you. I look forward to your reply.
Eliminating all grains was a wonderful step. But the food itself should be raw. Even the best no grain dry and canned foods are cooked, taking important nutrients away. If it is too difficult to prepare the raw food yourself, I would suggest prepackaged raw food, such as the BARF Diet. You can get this online.
As for colloidal silver, it may be that you were not giving them enough. Go up to two ounces each, and since it is only 10 ppm, maybe more. It will not hurt them, and be sure to continue the probotics. The ideal ppm is 20, so up it as much as you can. And yes, squirt it in their mouths besides putting it in their water. Frankly, this should do the trick.
Please keep me posted.